VACF and fix style vector


I'm trying to calculate the VACF as per the manual;

compute 1 lithium vacf
fix 1 lithium vector 10 c_1[4]
variable velocityacf equal dt*trap(f_1)

But I receive and error;

ERROR: Invalid fix style (../modify.cpp:714)

I can't find anything in the manual about compiling extra packages in order to use this fix style and the link for the fix style vector isn't working on the website.

Any guidance would be most appreciated,

Dr. Chris Eames
UoBath, UK